Several considerations relative to the effectiveness of powdered activated carbon as the sole adsorbent for removal of organics are the same as granular activated carbon. General speaking, we can expect the same relative selectivity to remove impurities what we do not required. The exceptions are those that depends upon the nature of the carbon surface because compared with granular activated carbon, powdered activated carbon usually has different feature.
Several considerations relative to the effectiveness of powdered activated carbon as the sole adsorbent for removal of organics are the same as granular activated carbon. General speaking, we can expect the same relative selectivity to remove impurities what we do not required. The exceptions are those that depends upon the nature of the carbon surface because compared with granular activated carbon, powdered activated carbon usually has different feature. Cheaper materials are most often used for producing, and the product usually has a much lower density. Powdered activated carbon is generally quite friable, but this is not important because it is usually used on a once-through basis. If friable powdered activated carbon were to be regenerated, high losses undoubtedly would be observed.
The application of powdered activated carbon is different from that of granular activated carbon, the type of reactor used, the relative importance of adsorption kinetics, and the relative nature of competitive adsorption effects.
Powdered activated carbon is generally used in water treatment industry at the rapid mix unit, which disperses it rapidly. What the flocculation and sedimentation units usually follows to provide the necessary delay for many organics for adsorbing. There are other possible application including the intake and points after the rapid mix unit. The latter points are usually undesirable because of the reduced time of adsorption. If the carbon is added just ahead of the rapid filter, the contact time is only minutes or a fraction of a minute. powdered activated carbon may also be applied in solids contact upflow clarifiers. In these units the powdered activated carbon is suspended and the water being treated is passed through the suspension. High concentrations of powdered activated carbon can be used in this type of contactor, and recycling of the powdered activated carbon is possible.
When powdered activated carbon is applied so that it moves through the treatment units with the water flow, it tends to equilibrate with the concentration of organics in the water treatment plant effluent. By comparison, granular activated carbon at the adsorber inlet tends to equilibrate with the concentration in the water before it comes into contact with the carbon, while granular activated carbon at the adsorber outlet tends to equilibrate with the organics in the effluent. The net result is that granular activated carbon tends to equilibrate with higher concentrations than does powdered activated carbon. Because a much lower loading, i.e., mass of organic material adsorbed per mass of carbon, can be achieved at low equilibrium concentration as compared to high equilibrium concentration, more powdered activated carbon than granular activated carbon is generally needed to achieve the desired removal.
To date, powdered activated carbon has been used extensively in water treatment plants to remove organic compounds that cause offensive taste and odor (AWWA Committee Report, 1977). Very little has been done to evaluate its applicability to the removal of many other compounds of concern, such as THM precursors, TOC, specific chlorinated organics, etc. Some work has been performed using powdered activated carbon to remove THM's, but only one carbon was used and only one type of water was treated (U.S. Environmental Protection Agency, 1976a). Competitive effects and carbon type can significantly affect results. Therefore, tests should be conducted on other types of waters and with other carbons.
Adsorbing on powdered activated carbon is affected by other processes. Take an example, when applied in the rapid mix unit the powdered activated carbon is likely to become enmeshed in floc particles, thereby affecting the adsorption of certain compounds.
When powdered activated carbon is applied to water containing chlorine, the carbon will act as a reducing agent and destroy the chlorine. This increases the chlorine required to achieve a given level of disinfection. There is also some evidence that the reaction with chlorine has an adverse effect on the adsorption of organics (McGuire et al., 1978; Snoeyink et al., 1974). The significance of this problem needs further evaluation. It is also not known whether the chlorine that reacts with the carbon or the organic compounds on the carbon surface produces undesirable chlorinated organics that enter the treated water.
Adsorption of molecules that diffuse rapidly should reach equilibrium with the small powdered activated carbon particles in the water treatment plant. However, large molecules such as humic acid diffuse slowly, and it is very likely that adsorption equilibrium cannot be achieved in the available time.
Adsorbing molecules compete for sites on powdered activated carbon just as they do on granular activated carbon. Most of the same considerations apply to both materials, but one unique difference concerns the displacement of previously adsorbed molecules. In a granular activated carbon bed, weakly adsorbed material may be displaced, sometimes resulting in an effluent concentration that is greater than the influent concentration. When powdered activated carbon is used in a sludge blanket, such as in an upflow solids contact clarifier, it is expected to behave similarly, but when it is applied so that it moves through the treatment plant with the water being treated until it settles out, the effluent concentration will always be less than the influent concentration, although the percent that is removed will not be as high as when there is no competition.
Summary, Conclusions, and Recommendations
Lower loadings can be achieved on powdered activated carbon than on granular activated carbon in a fixed or fluidized bed adsorber. Thus, higher doses of powdered activated carbon are generally required to achieve equivalent results. powdered activated carbon is also difficult to regenerate, but, because it costs less, it may be economically justified in certain applications. In general, the same types of compounds are expected to adsorb on powdered activated carbon as on granular activated carbon. When powdered activated carbon is applied so that it is in contact with water only a short time, it very likely does not support biological activity. Also, when powdered activated carbon moves with the water being treated, effluent concentrations greater than the influent will not occur. However, when it is applied in upflow solids contact clarifiers, the time of contact between the carbon particles and the water is longer, and microbial growth may attach to the carbon particles.
More research is recommended to determine the conditions under which powdered activated carbon is most effective and to ascertain whether the reaction of powdered activated carbon (as well as for granular activated carbon) and its adsorbed compounds with predisinfectants will result in undesirable compounds in the treated water.